Fiberoptic-based chemical sensor devices are becoming established analytical tools for remote and in situ chemical sensing. The ideal optical sensor device must have the ability to measure the concentration of an analyte continuously through changes in the optical properties of the sensing reagent. For example, for oceanographic and environmental sampling, conventional methods for measuring nutrient distributions, species of geochemical interest, and toxic chemicals usually require cumbersome discrete sampling techniques. The required sample handling and processing techniques are often time-consuming and labor-intensive and are subject to contamination and storage problems. Because samples are often processed days or weeks after collection, these techniques are not well-suited for mapping distributions of chemical constituents in dynamic aquatic environments. Sensors are needed that can be deployed in situ for the rapid, remote measurement of chemical species in aquatic samples.
Many colorimetric or fluorometric techniques are irreversible because they form a tight binding complex or utilize reagents that generate an irreversibly colored adduct. Irreversible sensors can be used if they operate in an integrating mode; however, they must be replenished frequently with fresh sensing reagent. Accordingly, it is often difficult to reuse the same sensor probe for a large number of measurements or for the continuous measurement of an analyte.
Many fiberoptic-based chemical sensors have been developed to attempt to continuously measure the presence and concentration of an analyte or analytes in a moving process stream. Many devices use an immobilized reagent to render them specific for an otherwise optically undetectable analyte of interest. These devices are often called "optrodes." Most optrodes have been limited to reversible reagent chemistries, because irreversible reagent reactions will severely limit the lifetime of the optrode probe and cause calibration problems. Moreover, the optrodes cannot use the vast number of irreversible reagent chemistries that form complex formations and colored products upon reaction with an analyte. Moreover, the optrodes with reversible reagent chemistries have had difficulties regarding probe-to-probe reproducibility, limited dynamic ranges, reagent photolability and leaching, and slow response times.
One approach to this problem was developed by Luo et al., "Fiber-Optic Sensors Based On Reagent Delivery with Controlled-Release Polymers," Anal. Chem. 61:174-77 (1989). Luo et al. describe a polymeric delivery system that attempts to deliver fresh sensing reagents for extended periods using a reversible sensing reagent. The Luo et al. probe releases the reversible reagent, entrapped in an ethylene-vinyl acetate polymer matrix, to the probe tip upon contact with an aqeuous solution. Several problems noted by Luo et al. included the observation that when the polymer was below the fiber tip, the emerging light from the fiber was reflected by the polymer surface and part of this reflected light reentered the fiber, resulting in a systematic error in the signal. Further, the Luo et al. design resulted in the buildup of a large concentration gradient of dyes around the polymer.
Another attempt by Inman et al., "Pressurized Membrane Indicator System for Fluorogenic-Based Fiber-Optic Chemical Sensors," Analytica Chimica Acta, 217:249-62 (1989), describes a fluorogenic indicator that is forced through an ultrafiltration membrane into the analyte solution. The reaction product between the reagent and the analyte (in this case an indicator molecule and a target ion, respectively) occurs at the membrane/solution interface. Light from a bifurcated fiberoptic cable stimulates fluorescence and fluorescence emission from the membrane/solution interface is transmitted back up the cable to a photodiode detector. The Inman et al. sensor has an element of renewability by continually renewing the indicator (reagent) at the end of the fiberoptic probe. However, the buildup of the reaction product on the membrane/solution interface again limits the usefulness of this sensor probe.
Accordingly, there is a need in the art to develop a completely renewable reagent sensor probe that can utilize the full spectrum of irreversible reagent chemistries as well as reversible reagent chemistries.